A method is known, which is for forming a protrusion part connected to an object by cutting (i.e., shaping) the object without separating the protrusion part from the object. In shaping process, the protrusion part is generally shaped so as to be rolled back, in order to increase a cutting efficiency. For example, Patent Document 1 (JP 2009-32755 A corresponding to US 2009/0025222 A1) discloses a method for forming a platy heat radiation fin that is a protrusion part rolled back.
When the rolled-back protrusion part is used as a heat radiation fin, air between the radiation fins is difficult to be exchanged for outer air. Thus, a cooling capacity of the heat radiation fin may be relatively small.